We are currently isolating and purifying an apparently high molecular weight form of procollagen from fetal calf skin. SDS gel electrophoresis of this protein indicates that the molecular weight is approximately 200,000/chain. Reduction of the disulfide bridges results in two bands on SDS electrophoresis which migrate to the position of collagen beta chains. Enzymic digestion results in the formation of alpha chains in SDS gels. The principal problems to be addressed in the future are the final complete purification and the determination of the character of the non-collagenous extensions. Such studies will include amino acid analysis, determinations of carbohydrate content, and the number of disulfide bridges. The molecular weight of the intact procollagen will be determined by amino acid analyses (hydroxyproline content) and by sedimentation equilibrium studies.